1. Field of the Invention
This invention relates in general to a radiant cooling unit, and more particularly to a synthetic gas cooling apparatus in which thermal protection is provided to avoid inadvertent operational damage The hot gas producing process utilizes a fuel comprised of finely ground coal, coke, or even a liquid fuel which is combusted in a controlled temperature atmosphere. The resulting effluent includes not only synthetic gas but also an amount of dust or fly ash which is separated into cooled gaseous and solid components.
2. Description of the Prior Art
Numerous heat exchangers and gas cooling design components known in the art are employed in different environments and for different applications. Cooling of hot synthetic gas from a gasifier of the type contemplated here, and the finely divided solids, such as powdered fly ash or slag, carried by the gas has heretofore utilized radiation boilers which functionally raise steam in water wall tubes. In such a boiler or heat exchanger, the solidified slag and cooled synthetic gas are removed at the lower end of a downflow radiation boiler. In commercial size plants and installations however, the radiation boiler dimensions could become a limiting factor where a single entrained throughput flow of the hot syngas is handled.
It is of further note that various structures for heat exchangers such as radiant waste heat boilers, are utilized for transferring or recovering heat from a flow of hot pressurized gas that is to be cooled. Many of these structures are not readily adapted to large scale flow of syngas. Also, there are known waste heat boilers that are characterized by a relatively complex structure, one such being described in U.S. Pat. No. 4,377,132.
Such structures, because of excessive heat and high pressure conditions under which they normally operate, are highly susceptible to damage in the event of a minor equipment malfunction. Further, the gas generation process could take an unexpected step as a result of undesired accumulations of the solid material from the hot produced gaseous effluent.
For example, a critical operating problem can result when the solid particulate matter is not completely removed from the effluent stream by water quenching or the like. Thus, the particles, as they leave the gasifier disengagement zone, and prior to being discharged, will tend to accumulate. The eventual results will be a solid mass or barrier to gas flow control thereby interfering with or completely retarding the normal gas flow from the radiant heating section.
An additional problem also encountered in the configuration of such coolers is the propensity for corrosive action on the unit's cooler walls and surfaces. More particularly, the cooling unit's outer shell wall, when shielded from heating by a protective inner wall, promotes condensation of hot gases which contain an amount of corrosive elements. An example of the latter is hydrogen chloride. When the latter comes into contact with a condensed water, the resulting acid will soon adversely affect any of the metal parts of the cooler with which it comes into contact.
Toward avoiding serious damage to a radiant type cooler of the type contemplated, the present invention affords a degree of protection against thermal as well as corrosive damage. The primary protection is provided through a "double dip tube" arrangement as well as a constant flow purge system communicated with the gasifier unit. The double-dip tube arrangement is designed in such a manner that no damage to the vessel will result if any plugging occurs in the region of the vessel's gas outlet nozzle. Unlike previous designs, the disclosed embodiment will ensure that if plugging does occur in this region, the differential pressure across the water wall will be equalized. These water walls are only capable of withstanding very small differential pressures, typically in the range of 10 to 15 psi. If differential pressures greater than the design level are experienced, then a rupture of the water wall could result. This circumstance would expose the vessel shell to excessively high temperatures which could result in the vessel's shell being ruptured.
The disclosed constant purge system acts to protect the surface of the external water wall by constantly sweeping the outer surface thereof, as well as the internal surface of the enclosing shell. This action avoids or at least minimizes the possibility of a breaching of one or both of these walls as a result of sustained contact with the hot gas and/or corrosive acids.
Both functions of the purge system, although in constant use, are preferably actuated in response to a monitor or sensing means positioned within the cooler. Thus, the constant flow of purge gas can be increased as necessary to a pressure and volume that will preclude entry of hot gases into the elongated annulus defined between the water wall and the shell.
It is therefore an object of the invention to provide a syngas cooler structure that is adapted for safely conducting a high temperature, particle-laden gaseous stream.
A further object is to provide a unit of the type contemplated which can function without damage under operating conditions including a high temperature environment in which a large amount of solids is carried in the hot gaseous stream and which is disposed of prior to their accumulating in flow passages.
Another object is to provide a heat exchanger that will require only relatively brief shutdown periods as a result of solids accumulations or to needed repair due to corrosive action on exposed walls.